Corrosion-resistant low alloy steel



United States Patent Oj CORROSION-RESISTANT LOW ALLOY STEEL No Drawing. Application January 31, 1957 Serial No. 637,376

7 Claims. (Cl. 75123) This invention relates to corrosion-resistant low alloy steel and particularly to such a steel which is especially adapted to be cold worked and used in sheet form.

It has been found that two types of rust form on steel articles, one type being protective and the other nonprotective. The former develops on automobile body exteriors, for example, where paint is chipped, scratched or abraded from the steel surfaces. Although oxidation of this type produces discoloration of the steel, it can be controlled by polishing, waxing or touch-up painting. Even without such treatment this type of rust forms a protective coating which greatly retards further corrosion.

However, the second type of rust, frequently referred to as sheltered corrosion, occurs in hidden, inaccessible locations from which moisture cannot readily escape. This non-protective rust continues to eat into the metal. Certain alloying elements which, when added to iron, contribute to the development of a protective rust do not appear to decrease sheltered corrosion.

For example, copper, nickel and chromium may be included in low carbon steels to improve corrosionr'esistance under environmental conditions where protective rusts are formed. Under typical atmospheric conditions, where the steel surfaces are periodically exposed to the washing action of rainfall followed by complete drying, additions of these elements greatly enhance the protection aflorded by the rust. Hence many conventional high-strength, low alloy steels contain these elements in order to provide better corrosion resistance.)

As indicated above, however, the presence of copper, nickel and chromium in the usual small amounts does not appreciably improve corrosion resistance of low alloy steels under sheltered conditions'where the steel surfaces remain moist for long periods of time without benefit of washing action or complete drying. In such a constantly moist environment the rust formed is porous and nonprotective in nature, regardless of the presence of these alloying elements. This results from the fact that the soluble corrosion products are not washed away, and the steel is not heated sufficiently to drive out the moisture which is retained in the pores. These phenomena have been clearly demonstrated by controlled atmospheric corrosion tests.

Accordingly, a principal object of the present invention is to provide a low alloy steel which is characterized by improved corrosion resistance under the sheltered moisture conditions hereinbefore described. A further'o'bject of this invention is to provide a low alloy steel of this type which possesses sufiiciently low yield strength and high ductility so that it may be readily coldformed by conventional methods.

These and other objects are attained in accordance with the present invention with a low alloy steel, preferably sheet steel, containing small amounts of arsenic, molybdenum, antimony, tin and manganese, as well as carbon, phosphorus and silicon. Nitrogen also maybe present a very small quantity and further contributes tothe corrosion resistance of the steel.

Sulfur, which is generally preferred carbon range for sheet steel is found as an impurity in steel, does not improve resistance to corrosion and should be held to a minimum. The combination of these various elements, when they are added in small amounts to a low carbon rimmed or aluminum killed steel composition, significantly improves the oxidation resistance of the steel under conditions where washing action and periodic complete drying do not occur. Sheet steel of this type is especially useful for automobile bodies or the like because of its relatively high ductility and low yield strength which permit the steel to be readily drawn or stamped.

The carbon increases the corrosion resistance of the low alloy steel when present in amounts up to at least 1%. However, in sheet steel applications. where cold forming is necessary it is desirable to use appreciably less than 1% carbon since carbon raises the yield strength and reduces the ductility of steel as well as increasing its tensile strength and hardness. Difiiculties are therefore encountered in cold forming operations if the steel has too high a carbon content. Weldability also is somewhat reduced with increasing amounts of carbon. Accordingly, if the steel article is tobe cold formed it is advantageous to have a carbon content not in excessof approximately 0.2%. Although the steel may contain between about 0.03% to 1% carbon for some applications, the approximately Manganese likewise improves the corrosion resistance of the steel and combines with sulfur to reduce the deleterious elfects of sulfur on the physical properties of steel. The corrosion resistance of the steel is' particularly im proved when manganese is present in combination with arsenic, antimony and tin. At least 0.25%' manganese is' desirable for anti-corrosion properties, and the manganese content may be as high as about 1.5% for some applications. If too much manganese is employed, however, the yield strength of the steel is increased to an excessive extent. Hence for sheet steel to be cold formed, such as by deep drawing operations, a maximum manganese content of 0.8% is preferred. Sheet steel of this type generally should contain between about 0.3% and 1 0.8% manganese. It appears that approximately 0.25%

to 0.4% manganese is usually necessary to overcome the detrimental effects of the sulfur in the steel, depending on the amount of sulfur present, of course.

Small amounts of phosphorus markedly improve corrosion resistance but also measurably reduce the ductility mately 0.005% to 0.05%

and increase the yield strength of low alloy steel. Therefore, in applications where severe cold forming is required, phosphorus should 'not be present in amounts greater than about 0.05%. On the other'hand, in articles where high strength, as well as corrosion resistance is an important factor, the phosphorus content may be as high as 0.15%. As a result, we have found that phosphorus frequently may be employed in an amount ranging from 0.005 to 0.15%, but that it is desirable to use approxiphosphorus in steel which is to be cold formed. I

Molybdenum is very similar to manganese in its eifects on the corrosion resistance and physical properties of low alloy steel. It increasescorrosion resistance very sign'ifi-.

Y 'cfantly when present in combination with arsenic, anti mony andtin. With this combination of alloying elements molybdenum improves corrosion resistance much more effectively than does substantially increasing the manganese content. :At'the same time, proper amounts;

of molybdenum do not causethe steel to have undesirable physical properties; We have found that a minim1'iirif molybdenum content of about-0.05% is desirableQ-Itis is to be cold formed, it is preferable to add between 0.1% and 0.4% molybdenum, a molybdenum content of about 0.2% appearing to produce optimum results with respect to h combinati n of o sion e st nc and yie d strength. V i l V a The most important single constituent in the corrosionresistant low alloy steel described herein arsenic. This element has proved to be extremely effective, even invery small amounts, in improving the corrosion resistance of the steel. Although arsenic tends to reduce the ductility and increase the yield strength, as well as the hardness and tensile strength of the steel, it canbe used in amounts which are sufiiciently small so that its undesirable effects on physical properties are not significant. In general, arsenic may be presentin anamount ranging from about 0.02% to 0.5%. Although an arsenic content of only 0.02% has a measurable effect in reducing corrosion resistance, it is normally preferable to employ at least 0.05% of this element. The maximum arsenic content is governed to a considerable extent by the cost and availability of arsenic, as well as the fact that an excessive amount adversely affects physical properties of the steel. For most applications, it is preferable to use approxi mately 0.1% to 0.25% arsenic.

The antimony further contributes to the corrosion resistance of the steel. While its effect on corrosion resistance is not as pronounced as that of arsenic, it likewise does not appreciably affect the physical properties of the steel if present in proper amount. Antimony may be entirely omitted from steels used in certain applications, but it is advantageous to employ a small quantity of anti mony because it permits a reduction in the amount of arsenic which must otherwise be used. An antimony content of -about.0.02% to 0.5% has therefore proved to be satisfactory. However, for optimum results a 0.05% to 0.25% antimony range is preferred. Ifmore than 0.25% antimony is employed, the cost of the steel'is increased, of course; and the antimony begins to have a measurable effect on the physical properties of the steel.

As indicated above, tin also improves the corrosion resistance of the steel and, like many of the other constituents, it adversely influences the desired physical properties of the steel if used in excessive amounts. Consequently, a tin content of approximately 0.04% to 0.25% is satisfactory, while about 0.07% to 0.15% tin is normally preferred.

Silicon, which is always present in very small quantities in steel, moderately improves its corrosion resistance. While it is often necessary to use less than 0.1%silicon where severe cold forming is required, a silicon content as high as 0.5 may be satisfactory if the steel is to be only moderately cold worked. Rimmed steels, such as those used for automobile body stock, normally contain approximately 0.01% to 0.1% silicon.

Sulfur, like silicon, is normally found in steel as an impurity. However, since sulfur does not increase corrosion resistance and has undesirable eflects on the yield strength, ductility, and hot and cold forming properties of the steel, it should not be present in amounts greater than about 0.05%. For most applications the sulfur content should not exceed 0.02%, while the usual minimum amount is approximately 0.001%.

Another typical impurity which is usually present in.

steel is nitrogen. Although this element is beneficial to the steel with respect to corrosion resistance, it embrittles steel and drastically reduces its ductility and raises its yield strength. Nitrogen 'also produces strain aging of the steel. As a result, a high nitrogen'content is considered disadvantageous from the standpoint of steel fabrication because it causes surface defects in'cold drawn parts. It is therefore desirable to have nitrogen present in a very small amount which is sulncient to improve the corrosion resistance of the steel without excessively ina creasing its yield strength. a

iftequently aluminum is addedto sheet steel composi- 4 tions to combine with a portion of the nitrogen present and/or to deoxidize the steel. Although higher percentages of aluminum are often added, normally 0.2% is considered the maximum amount of residual aluminum which should be present in the low alloy steel described herein. Typically 0.03% residual aluminum is used to combine with the nitrogen, and this aluminum or the amount in solution usually should not exceed about 0.1%. However, an approximately equal amount may bepresent in the steel as aluminum oxide.

Other elements, such as nickcLvanadium and small amounts of chromium, titanium and zirconium, for example, may be added to the steel without significantly decreasing its corrosion resistancein instances where it is advantageous to add such elements to improve other physical properties. It should be noted, however, that the alloy described and claimed herein is not a stainless steel but is a low alloy steel having corrosion resistance which measurably superiorfto that possessed by the low carbon, low alloy sheet steels presentlycommercially availablef The term "low alloy steel,"as used herein, refers to steel which generally contains hormone than about 5% or 6% total alloying elements. A

It will also be noted'that' this high corrosion resistance obtained largely without using relatively scarce. strategic'rrie tals."'The particular combination of alloying elements described, rather than any one or two of these elements, produces this result. At the same time, this combination of constituents provides a low alloy steel which h as the neces sar y high ductility and low yield point to p err nit'itto be advantageouslyused as sheet material for drawing or other cold forming operations.

In conducting tests to evaluate the corrosion resistance of this low alloy steel, bare panels were suspended in a constant temperature cabinet at 125 F. and were rusted by humid air passing over them. The humidity wascycled between and 10% for eight-hour periods Each panel was wetted' with a dilute salt solution every day during a twenty-day test period. It was observed that each humidity cycle produced a new layer of rust. These tests indicated that conventional drawing quality steels corroded as much as 100% faster than the low alloy steel described herein.

As stated above, this steel composition possesses high resistance to atmospheric corrosion in sheltered areas where the steel remains moist for extended periods of time. These sheltered areas include isolated locations the scope'of the invention is not to be limited thereby except as defined in the following claims.

We claim:

1. A corrosion-resistant low alloy steel comprising about 0.03% to 1% carbon, 0.25% to 1.5% manganese, 0.05% to 0.8% molybdenum, 0.02% to0.5% arsenic, 0.02% to 0.5 antimony, 0.04% to 0.25% tin, and the balance substantially all iron. 7 7

2. A low alloy steelcharacterized by improved corrosion resistance under conditions of constant high humidity, said steel consisting essentially of about 0.03% to 1% carbon, 0.25% to 1.5% manganese, 0.05% to 0.8% molybdenum, 0.02 to 0.5% arsenic, 0.02% to 0.5% antimony, 0.04% to 0.25 tin, phosphorus not in excess of 0.151%, silicon not inexcess of 0.5%, aluminum not in a V to 0.25% tin, 0.005% to 0.15% phosphorus, 0.01% to 0.5% silicon, aluminum not in excess of 0.2%, and the balance substantially all iron.

4. A corrosion-resistant low alloy sheet steel characterized by excellent cold forming properties, said sheet steel consisting essentially of about 0.04% to 0.2% carbon, 0.3% to 0.8% manganese, 0.1% to 0.4% molybdenum, 0.05% to 0.25% arsenic, 0.05% to 0.25% antimony, 0.07% to 0.15 tin, and the balance substantially all iron.

5. A corrosion-resistant sheet metal panel for an automobile body, said panel being formed of a low alloy steel characterized by high ductility and low yield strength, said steel consisting essentially of about 0.04% to 0.2% carbon, 0.3% to 0.8% manganese, 0.1% to 0.4% molybdenum, 0.1% to 0.25% arsenic, 0.05% to 0.25% antimony, 0.07% to 0.15 tin, phosphorus not in excess of 0.05%, silicon not in excess of 0.5%, sulfur not in excess of 0.05%, and the balance substantially all iron.

'6. An automobile body having sheet metal panels formed of a corrosion-resistant low alloy steel characterized by high ductility and low yield strength, said steel consisting essentially of about 0.04% to 0.2% carbon, 0.3% to 0.8% manganese, 0.1% to 0.4% molybdenum,

0.1 to 0.25% arsenic, 0.05% to 0.25% antimony, 0.07% to 0.15% tin, 0.005% to 0.05 phosphorus, 0.01% to 0.1% silicon, 0.001% to 0.02% sulfur, aluminum not in excess of 0.2%, nitrogen in a very small amount which is eflFective to improve corrosion resistance without excessively increasing yield strength, and the balance iron plus incidental impurities.

7. A corrosion-resistant sheet metal panel for an automobile body, said panel being formed of a low alloy steel characterized by high ductility and low yield strength, said steel consisting essentially of about 0.04% to 0.2% carbon, 0.3% to 0.8% manganese, 0.1% to 0.4 molybdenum, 0.1% to 0.25% arsenic, 0.07% to 0.15% tin, 0.005% to 0.15% phosphorus, 0.01% to 0.5% silicon, and the balance substantially all iron.

OTHER REFERENCES Metals Handbook, 1948 ed., page 308; published by the American Society for Metals, Cleveland, Ohio. 

1. A CORROSION-RESISTANT LOW ALLOY STEEL COMPRISING ABOUT 0.03% TO 1% CARBON, 0.25% TO 1.5% MANGANESE, 0.05% TO 0.8% MOLYBDENUM, 0.02% TO 0.5% ARSENIC, 0.02% TO 0.5% ANTIMONY, 0.04% TO 0.25% TIN, AND THE BALANCE SUBSTANTIALLY ALL IRON. 